1. Field of the Invention
This invention relates to wireless communication systems, and more particularly to cellular communication systems.
2. Description of Related Art
Mobile radio communication systems generally comprise two components: fixed base stations and radio system users (fixed or mobile). The fixed base stations provide an interface between the radio system users and users of other wired or wireless systems. The base stations typically are geographically deployed to optimize service for the system users throughout a service area. A base station's coverage area is defined as a composite of all locations in which a system user can communicate with the base station. The coverage area is limited both by distances and obstacles (natural and man-made) which inhibit the propagation of radio signals transceived by a base station. Fixed base stations are typically deployed so that the composite of their respective coverage areas define a desired service area for a given system.
Adjacent base stations often have overlapping coverage areas. Consequently, users of adjacent base stations often will interfere with each other when they attempt to concurrently transceive different information using the identical frequency channels. Some prior art radio communication systems, referred to as "simulcast" or "broadcast" systems, solve this interference problem by forcing every fixed base station to transmit the identical information using identical frequency channels. Thus, rather than interfere with each other, the prior art simulcast base stations tend to reinforce each other. Under many topographical conditions, transmissions received from several simulcast base stations have improved signal strengths as compared to transmissions received from a single base station. Consequently, the prior art simulcast systems are able to transmit signals into areas that suffer from large transmission path losses.
Disadvantageously, the prior art simulcast systems have relatively low net data transmissions rates. The capacity of the prior art simulcast systems is limited because a plurality of base stations are used to transmit identical information. For example, in some prior art simulcast paging systems the total bit rate supportable for an entire metropolitan area is typically only 2,400 bits per second (bps). Such limited capacity has heretofore been adequate for supporting numeric paging messages in which messages primarily comprise a single telephone number. However, new services such as alphanumeric paging (where messages comprise a large number of text characters) require increased system capacity. Consequently, many simulcast paging systems are approaching capacity and cannot support such services.
A more common means for avoiding interference between adjacent base stations is to assign different frequencies to each base station and allow them to transceive different information. This approach is used in the prior art cellular communication systems. In contrast to the prior art simulcast paging systems, each base station in a cellular paging system transmits different streams of information using different frequencies. Thus, cellular systems are being developed to overcome the shortcomings of the prior art simulcast systems. For example, Narrowband Personal Communications Systems (or "NPCS") are being developed to provide a new class of messaging services and instrumentation having improved efficiency and greater functionality in comparison with the prior art messaging or paging systems. One example of a presently contemplated NPCS system is described in the personal Air Communication Technology (pACT) system specification, release 1.1, hereinbelow referred to as the "pACT specification", distributed by the American Telephone and Telegraph (AT&T) Wireless Services company.
Although the new cellular messaging and paging systems promise increased capacity and a wider range of services than has previously been available, they typically are deployed in environments where the available frequency spectrum is quite limited. For example, the pACT specification contemplates deployment scenarios where a typical system is allocated merely two or three frequency channels for forward link transmissions (transmissions from base stations to subscriber units). As described in a related patent application (commonly assigned co-pending application, U.S. Ser. No. 08/533,664, hereinafter referred to as "the related application"), when all frequencies allocated to a system have been used it is necessary to begin reusing the frequencies. The frequency reuse process requires careful assignment of frequencies to base stations and radio communication service areas. This assignment process is referred to as "frequency planning" or "cell planning."
FIG. 1 is a block diagram of a prior art cell configuration showing the problems associated with frequency reuse and frequency planning. Clusters of seven cells (modeled as hexagons for ease of understanding) form "cell groups" or "frequency reuse groups" 1, indicated by bold lines. In the example shown, seven frequencies are used within each cell group 1, and then reused in adjacent cell groups 1. Within each cell group 1, the pattern of frequency distribution is normally the same. Thus, the center cell 2a of the central cell group 1a shown uses the same frequency as the center cell 2b of the adjacent cell group 1b. Because the number of available frequencies is often limited the exercise of defining which frequencies are used by which base station has become increasingly complex.
Because frequencies are reused, two cells or base stations operating on the same frequency, though separated geographically, may interfere with each other. This is known as "co-channel interference." In many cases path loss conditions may cause the difference between the desired carrier power and the interference (known as the ratio "C/I") to be insufficient for reliable receiver performance. The overall effect of co-channel interference is to create areas within a cell where no good coverage is possible. In the case of seven total frequencies, these bad locations may comprise 40% or more of a typical cell. The traditional way to mitigate co-channel interference in FDMA systems is to allocate a larger number of frequencies to the service and to devise sparse reuse patterns. However, this method cannot be used when only a small number of frequencies, such as three, are available.
As described in the related application, an alternative method of solving the problems associated with co-channel interference in systems having only a small number of frequencies is to time-share the available frequencies between base stations. Frequency reuse is enhanced by synchronizing cell transmit/receive base stations in a cellular system to a common time base, and then sharing the available frequencies via allocated time slots. That is, cells or base stations using the same frequency that potentially interfere with each other are activated only during selected time intervals while same-frequency cells nearby are deactivated. The deactivated cells are then in turn activated while previously activated same-frequency cells nearby are deactivated. Thus, frequency planning is simplified in systems that do not otherwise have a sufficient number of unique frequencies to provide adequate spatial separation between same frequency base stations.
As taught in the related application, a number of different time slot allocation schemes may be used. For example, FIG. 2 is a timing diagram showing equal time slot allocations between two potentially interfering cells or base stations. The horizontal dimension represents time. In the example shown, a first potentially interfering base station X is activated during a first time period Ta, while a second potentially interfering base station Y is de-activated during the same first time period Ta. In a second time period Tb, the first base station X is de-activated while the second base station Y is activated. In a third time period Ta', the base stations reverse state again, as in the first time period Ta, and so forth in the fourth time period Tb'. FIG. 3 is a timing diagram showing equal time slot allocations among three potentially interfering base stations X, Y, and Z during cyclical time periods Ta, Tb, and Tc. Although only a 3-time slot cycle is shown, 4 or 5 or more time slots could be used, with "extra"time slots being allocated to one or more of the base stations based on relative usage among the base stations. Alternatively, the durations of time slots Ta, Tb, and Tc can vary based on relative usage.
Disadvantageously, one artifact of time-sharing base stations in the manner shown in FIGS. 2-3 is that each base station is allowed to transmit on an assigned frequency only during designated time periods. Consequently, the base stations are idle a significant percentage of the time. For example, the base stations of FIG. 2 are idle 50% of the time, while those of FIG. 3 are idle 66% of the time. Considering the expense associated with the purchase, deployment and maintenance of cellular systems, idle base stations represent undesirable overhead costs to system providers. Therefore, it is desirable to provide a time-shared cellular communication system which uses a small number of frequencies, substantially reduces significant co-channel interference, yet which provides efficient use of otherwise "idle" time-shared base stations. The present invention provides such a cellular communication system by sharing the "idle" time-shared base stations between two or more radio communication systems.